Prior to the inventions described in the aforementioned related patent applications, the liquid spray application of coatings, such as lacquers, enamels and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason there has been a great deal of emphasis placed on the development of new coatings technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged as having met most but not all of the performance and application requirements, and at the same time meeting emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) water-borne solutions, (d) non-aqueous dispersions, and (e) high solids coatings. Each of these technologies has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that were initially expected.
Powder coatings, for example, while providing ultra low emission of organic vapors, are characterized by poor gloss or good gloss with heavy orange peel, poor distinctness of image gloss (DOI), and poor film uniformity. Moreover, to obtain even these limited performance properties generally requires excessive film thicknesses and/or high curing temperatures. Pigmentation of powder coatings is often difficult, requiring at times milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coating often requires its complete cleaning, because of dust contamination of the application equipment and finishing area.
Water-borne coatings are very difficult to apply under conditions of high relative humidity without serious coating defects. These defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes unfortunately often result. Additionally, water-borne coatings are not as resistant to corrosive environments as are the more conventional solvent borne coatings.
Coatings applied with organic solvents at high solids levels avoid many of the pitfalls of powder and water-borne coatings. However, in these systems the molecular weight of the polymer has been decreased and reactive functionality has been incorporated therein so that further polymerization and crosslinking can take place after the coating has been applied. It has been hoped that this type of coating will meet the ever-increasing regulatory requirements and yet meet the most exacting coatings performance demands. However, there is a limit as to the ability of this technology to meet the performance requirement of a commercial coating operation. Present high solids systems have difficulty in application to vertical surfaces without running and sagging of the coating. Often, they are also prone to cratering and pin holing of the coating. If they possess good reactivity, they often have poor shelf and pot life. However, if they have adequate shelf stability, they cure and/or crosslink slowly or require high temperature to effect an adequate coating on the substrate.
Clearly, what is needed is an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymer and coatings compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent borne coatings applications and performance while reducing the environmental concerns to an acceptable level. Such a coating system could meet the requirements of shop- and field-applied liquid spray coatings as well as factory-applied finishes and still be in compliance with environmental regulations.
Such a needed diluent has now been found and is discussed in the aforementioned related applications which teach, among other things, the utilization of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersion coating compositions to dilute these compositions to application viscosity required for liquid spray techniques.
U.S. patent application Ser. No. 133,068, filed Dec. 21, 1987, to Hoy, et al., discloses processes and apparatus for the liquid spray application of coatings to a substrate that minimize the use of environmentally undesirable organic diluents. The broadest process embodiment of that application involves:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(a) at least one polymeric compound capable of forming a coating on a substrate; and PA1 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray application; and PA1 (a) at least one polymeric component capable of forming a coating on a substrate; and PA1 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; and PA1 (a) at least one polymeric component capable of forming a coating on a substrate; and PA1 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application;
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon.
That application is also directed to a liquid spray process in which at least one active organic solvent (c) is admixed with (a) and (b) above prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid is supercritical carbon dioxide. The process employs an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus contains:
(1) means for supplying at least one polymeric compound capable of forming a continuous, adherent coating;
(2) means for supplying at least one active organic solvent;
(3) means for supplying supercritical carbon dioxide fluid;
(4) means for forming a liquid mixture of components supplied from (1)-(3); and
(5) means for spraying said liquid mixture onto a substrate.
The apparatus may also provide for (6) means for heating any of said components and/or said liquid mixture of components. U.S. patent application Ser. No. 133,068 demonstrates the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute the compositions to application viscosity required for liquid spray techniques. It further demonstrates that the method is generally applicable to all organic solvent-borne coatings systems.
Copending U.S. application Ser. No. 218,910, filed Jul. 14, 1988, is directed to a liquid coatings application process and apparatus in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce to application consistency, viscous coatings compositions to allow for their application as liquid sprays. The coatings compositions are sprayed by passing the composition under pressure through an orifice into the environment of the substrate.
In particular, the process of U.S. application Ser. No. 218,910 for liquid spray
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray.
U.S. application Ser. No. 218,895, filed Jul. 14, 1988, is directed to a process and apparatus for coating substrates by a liquid spray in which 1) supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations, 2) the mixture of supercritical fluid and coating formulation is passed under pressure through an orifice into the environment of the substrate to form the liquid spray, and 3) the liquid spray is electrically charged by a high electrical voltage relative to the substrate.
In particular, the process of U.S. application Ser. No. 218,895 for electrostatic liquid spray application of coatings to a substrate comprises:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray; and
(3) electrically charging said liquid spray by a high electrical voltage relative to the substrate and electric current.
The use of supercritical fluids as a transport medium for the manufacture of surface coatings is well known. German patent application 28 53 066 describes the use of a gas in the supercritical state as the fluid medium containing the solid or liquid coating substance in the dissolved form. In particular, the application addresses the coating of porous bodies with a protectant or a reactive or nonreactive decorative finish by immersion of the porous body in the supercritical fluid coupled with a pressure drop to effect the coating. The most significant porous bodies are porous catalysts. However, the applicant characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, describe forming a supercritical solution which includes a supercritical fluid solvent and a dissolved solute of a solid material and spraying the solution to produce a "molecular spray." A "molecular spray" is defined as a spray "of individual molecules (atoms) or very small clusters of the solute." The Smith patents are directed to producing fine films and powders. The films are used as surface coatings.
Because of its relevancy to the present invention, a brief discussion of supercritical fluid phenomena is believed to be warranted.
Supercritical fluid phenomenon is well documented, see pages F-62-F-64 of the CRC Handbook of Chemistry and Physics, 67th Edition, 1986-1987, published by the CRC Press, Inc., Boca Raton, Fla. At high pressures above the critical point, the resulting supercritical fluid, or "dense gas", will attain densities approaching those of a liquid and will assume some of the properties of a liquid. These properties are dependent upon the fluid composition, temperature, and pressure. As used herein, the "critical point" in the transition point at which the liquid and gaseous states of a substance merge into each other and represents the combination of the critical temperature and critical pressure for a given substance. The "critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquefied by an increase in pressure. The "critical pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
The compressibility of supercritical fluids is great just above the critical temperature where small changes in pressure result in large changes in the density of the supercritical fluid. The "liquid-like" behavior of a supercritical fluid at higher pressures results in greatly enhanced solubilizing capabilities compared to those of the "subcritical" compound, with higher diffusion coefficients and an extended useful temperature range compared to liquids. Compounds of high molecular weight can often be dissolved in the supercritical fluid at relatively low temperatures. An interesting phenomenon associated with supercritical fluids is the occurrence of a "threshold pressure" for solubility of a high molecular weight solute. As the pressure is increased, the solubility of the solute will often increase by many orders of magnitude with only a small pressure increase. The solvent capabilities of the supercritical fluid, however, are not essential to the broad aspects of the present invention.
Near-supercritical liquids also demonstrate solubility characteristics and other pertinent properties similar to those of supercritical fluids. The solute may be a liquid at the supercritical temperatures, even though it is a solid at lower temperatures. In addition, it has been demonstrated that fluid "modifiers" can often alter supercritical fluid properties significantly, even in relatively low concentrations, greatly increasing solubility for some solutes. These variations are considered to be within the concept of a supercritical fluid as used in the context of this invention. Therefore, as used herein, the phrase "supercritical fluid" denotes a compound above, at, or slightly below the critical temperature and pressure (the critical point) of that compound.
Examples of compounds which are known to have utility as supercritical fluids are given in Table 1.
TABLE 1 ______________________________________ EXAMPLES OF SUPERCRITICAL SOLVENTS Critical Boiling Temper- Critical Critical Point ature Pressure Density Compound (.degree.C.) (.degree.C.) (atm) (g/cm.sup.3) ______________________________________ CO.sub.2 -78.5 31.3 72.9 0.448 NH.sub.3 -33.35 132.4 112.5 0.235 H.sub.2 O 100.00 374.15 218.3 0.315 N.sub.2 O -88.56 36.5 71.7 0.45 Xenon -108.3 16.6 57.6 0.118 Krypton -153.2 -63.8 54.3 0.091 Methane -164.00 -82.1 45.8 0.2 Ethane -88.63 32.28 48.1 0.203 Ethylene -103.7 9.21 49.7 0.218 Propane -42.1 96.67 41.9 0.217 Pentane 36.1 196.6 33.3 0.232 Methanol 64.7 240.5 78.9 0.272 Ethanol 78.5 243.0 63.0 0.276 Isopropanol 82.5 235.3 47.0 0.273 Isobutanol 108.0 275.0 42.4 0.272 Chlorotrifluoro- -31.2 28.0 38.7 0.579 methane Monofluoromethane -78.4 44.6 58.0 0.3 Cyclohexanol 155.65 356.0 38.0 0.273 ______________________________________
Due to the low cost, environmental acceptability, non-flammability and low critical temperature of carbon dioxide, supercritical carbon dioxide fluid is preferably used with the coating formulations. For many of the same reasons, nitrous oxide (N.sub.2 O) is a desirable supercritical fluid for admixture with the coating formulations. However, any of the aforementioned supercritical fluids and mixtures thereof are to be considered as being applicable for use with the coating formulations.
The solvency of supercritical carbon dioxide is substantially similar to that of a lower aliphatic hydrocarbon and, as a result, one can consider supercritical carbon dioxide as a replacement for the hydrocarbon solvent of a conventional coating formulation. In addition to the environmental benefit of replacing hydrocarbon solvents with supercritical carbon dioxide, there is a safety benefit also, because carbon dioxide is non-flammable.
Due to the solvency of the supercritical fluid with the coating formulations, a single phase liquid mixture is formed which is capable of being sprayed by airless spray techniques.
Coating formulations are commonly applied to a substrate by passing the coating formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating formulation and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams are used to modify the shape of the spray. The coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid coating formulation heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids atomization.
In essentially every process in which a mixture is prepared for a particular purpose, the constituents of that mixture usually need to be present in particular, proportionated amounts in order for the mixture to be effective for its intended use. In the aforementioned related patent applications, the underlying objective is to reduce the amount of organic solvent present in a coating formulation by the use of supercritical fluid. Understandably, with this objective in mind, it is generally desirable to utilize as much supercritical fluid as possible while still retaining the ability to effectively spray the liquid mixture of coating formulation and supercritical fluid and also obtain a desirable coating on the substrate. Accordingly, here too, it is particularly preferred that there be prescribed, proportionated amounts of supercritical fluid and of coating formulation present in the liquid coating mixture to be sprayed.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the coating formulation. This practical upper limit is generally recognizable when the admixture containing coating formulation and supercritical fluid breaks down from one phase into two fluid phases.
To better understand this phenomenon, reference is made to the phase diagram in FIG. 1 wherein the supercritical fluid is supercritical carbon dioxide fluid. In FIG. 1, the vertices of the triangular diagram represent the pure components of an admixed coating formulation which for the purpose of this discussion contains no water. Vertex A is an organic solvent, vertex B is carbon dioxide, and vertex C represents a polymeric material. The curved line BFC represents the phase boundary between one phase and two phases. The point D represents a possible composition of a coating formulation in which supercritical carbon dioxide has not been added. The point E represents a possible composition of an admixed coating formulation, after admixture with supercritical carbon dioxide.
Thus, after atomization, a majority of the carbon dioxide vaporizes, leaving substantially the composition of the original coating formulation. Upon contacting the substrate, the remaining liquid mixture of the polymer and solvent(s) component(s) will flow, i.e., coalesce, to produce a uniform, smooth film on the substrate. The film forming pathway is illustrated in FIG. 1 by the line segments EE'D (atomization and decompression) and DC (coalescence and film formation).
However, the amount of supercritical fluid, such as supercritical carbon dioxide, that can be mixed with a coating formulation is generally a function of the miscibility of the supercritical fluid with the coating formulation as can best be visualized by referring to FIG. 1.
As can be seen from the phase diagram, particularly as shown by arrow 10, as more and more supercritical carbon dioxide is added to the coating formulation, the composition of the admixed liquid coating mixture approaches the two-phase boundary represented by line BFC. If enough supercritical carbon dioxide is added, the two-phase region is reached and the composition correspondingly breaks down into two fluid phases. Sometimes, it may be desirable to admix an amount of supercritical fluid (in this case, supercritical carbon dioxide) which is even beyond the two phase boundary. Generally, however, it is not preferable to go much beyond this two phase boundary for optimum spraying performance and/or coating formation.
In addition to avoiding the two-phase state of the supercritical fluid and the coating formulation, proper proportionation is also desirable to provide optimum spraying conditions, such as, formation of desired admixed viscosity, formation of desired particle size, formation of desired sprayed fan shape, and the like.
Accordingly, in order to spray liquid coating formulations containing supercritical fluid as a diluent on a continuous, semi-continuous, and/or an intermittent or periodic on-demand basis, it is necessary to prepare such liquid coating formulations in response to such spraying by accurately mixing a proportioned amount of the coating formulation with the supercritical fluid. However, the compressiblity of supercritical fluids is much greater than that of liquids. Consequently, a small change in pressure results in large changes in the density of the supercritical fluid.
The compressibility of the supercritical fluids causes the flow of these materials, through a conduit and/or pump, to oscillate or fluctuate. As a result, when mixed with the coating formulation, the proportion of supercritical fluid in the resulting admixed coating formulation also correspondingly oscillates or fluctuates instead of being uniform and constant. Moreover, the compressibility of liquid carbon dioxide at ambient temperature is high enough to cause flow oscillations and fluctuations to occur when using reciprocating pumps to pump and proportion the carbon dioxide with the coating formulation to form the admixed coating formulation. This particularly occurs when the volume of liquid carbon dioxide in the flow path between the pump and the mixing point with the coating formulation is too large. The oscillation can be promoted or accentuated by any pressure variation that occurs during the reciprocating pump cycle.
In an embodiment discussed in a number of the aforementioned related patent applications, an apparatus is disclosed for pumping and proportionating a non-compressible fluid, i.e., a coating formulation with a compressible fluid, liquid carbon dioxide, for example, in order to prepare the ultimate mixture to be sprayed comprised of the coating formulation and the carbon dioxide in its supercritical state. In that embodiment, volumetric proportionating of the coating formulation stream and the liquid carbon dioxide stream is carried out by means of reciprocating pumps which displace a volume of fluid from the pump during each one of its pumping cycles. One reciprocating pump is used to pump the coating formulation which is slaved to another reciprocating pump which is used to pump the liquid carbon dioxide. The piston rods for each pump are attached to opposite ends of a shaft that pivots up and down on a center fulcrum. The volume ratio is varied by sliding one pump along the shaft, which changes the stroke length.
However, liquid carbon dioxide is relatively compressible at ambient temperature, the temperature at which it is typically stored in a pressurized container. Such compressibility may undesirably cause fluctuations and oscillations of the amount of carbon dioxide that is present in the admixed coating formulation that is to be sprayed. This occurs due to the incompatible pumping characteristics of the relatively non-compressible coating formulation and the relatively compressible liquid carbon dioxide. With the coating formulation, pressure is immediately generated in the reciprocating pump as soon as its volume is displaced. Inasmuch as the liquid carbon dioxide is substantially compressible, a larger volume is needed to be displaced in order to generate the same pressure. Because mixing occurs when the flow of the coating formulation and of the liquid carbon dioxide are at the same pressure, the flow rate of carbon dioxide lags behind the flow rate of the coating formulation.
This oscillation is accentuated if the driving force operating the pump varies during the operating cycle, such as an air motor changing direction during its cycle. Thus, if the driving force declines, the pressure in the coating formulation flow declines even more rapidly, due to its non-compressibility, than the pressue in the liquid carbon dioxide flow, due to its being compressible.
Accordingly, the pressures generated in both flows may be out of phase during the pumping cycle, such that the proportion of carbon dioxide in the mixture to be sprayed also varies. This oscillation is made even more severe if cavitation also occurs in the carbon dioxide pump due to vapor formation as the pump fills.
While some of these fluctuation and oscillation problems have been suppressed by refrigerating the liquid carbon dioxide to low temperatures, such as below 10.degree. C., and even below 0.degree. C., prior to its entering the reciprocating pump, a need still exists to avoid substantially all inaccuracies that may be present in the proportionation of the non-compressible coating formulation and the compressible liquid carbon dioxide to form the desired admixture. Indeed, a need exists to provide a means to accurately proportion any compressible fluid with a non-compressible fluid.